A gap of 6 gallons is excessive and likely your float is not reaching the top of the fuel tank when full. I normally see 2 or 3 gallons difference between maximum indicated and full when installing Skyview systems in RV's. While you could verify the float arm is bent correctly according to the plans such that it reaches all the way to the top surface of the fuel tank, there is going to be some limit to fuel load due to the fact there is only one sender and it is at the bottom of a sloping (wing dihedral) tank. My RV4 indication tops at 14 gallons in a 16 gallon tank. The Dynon indicates 14+ at values above 14 gallons but is accurate as fuel burns to below that level. The fuel computer function is very accurate. The only way I know to have the tank capacity indicate correctly from empty to full is to either use a properly designed capacitive probe or add a float to the outboard side of the tank and bus the sensors together. That is an awful lot of work. Check the correct travel of the float first.
